The PE spectra of the 4,5-dihydro-3H-pyrazoles (I; X,R given:CH2,H; CHOH,Me), of 2,5-dihydro-1,3,4-oxadiazole (II), and 2,5-dihydro-1,3,4-thidiazole (III) have been recorded. Based on HAM/3, MNDO, AM1, and PM3 calcns., the ionization potentials have been assigned to MOs. The gas-phase thermolyses of I, II, and III have been studied by PE-controlled gas anal. Extrusion of mol. nitrogen leads to reactive species which cyclize to three-membered rings of different stability. At higher temps. and in flash vacuum pyrolysis, consecutive reactions may lead to smaller acyclic mols. The cyclopropanol (IV), obtained by denitrogenation of I(X=CHOH,R=Me), is thermally rather stable. The PE spectrum of IV has been recorded and analyzed.